Terrorism, by definition, intends to create irrational fear — a mystique around the terrorists and victims that enables militant organizations to project far more power than their limited resources would otherwise allow. Peter Bergen’s achievement in his new book, “United States of Jihad,” is demystifying the domestic jihadis that have played such a central role in American politics since 9/11. In doing so, he reveals another truism: Sometimes, banal realities are more worrisome than exaggerated fear-mongering.

According to Bergen’s statistics, 330 people have been charged with jihadi terrorism in the United States since 9/11. Eighty percent were American citizens or permanent residents; most were well educated, one-third or so are married. Nearly 20 percent are women and the average age is 29. The vast majority of these people share little with the master terrorists of Hollywood nightmares. Rather, as Bergen notes, “they are ordinary Americans.”

Bergen writes about these American jihadis with the dexterous prose of a career journalist and the assumption that whatever crimes they ultimately committed, these people did not begin life as mass murderers. Sometimes, this approach raises more questions than it provides answers. Why, for example, did Nidal Hasan decide to open fire on his fellow soldiers at Fort Hood, Texas, but his first cousin, Nader, who was raised in a similar environment only blocks away, became a family man and respected lawyer in northern Virginia?

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On the one hand, Nidal was an Army officer clearly disturbed by the prospect of deploying to Afghanistan to support a war he opposed deeply. He was also in touch with Anwar al-Awlaki — himself an American citizen who had joined al Qaeda — and the jihadis’ single most effective English language propagandist. Nader, however, dismissed these explanations and offered a more prosaic account for his cousin’s behavior: He had become totally social isolated, with “no wife, no children, no parents, no friends.”

In the end, Nader argued, Nidal “went postal. And he called it Islam. He sucked every Muslim into his suicidal plan.”

Not every American jihadi fits that mold, but the statistical and psychological normalcy of would-be jihadis creates a huge problem for law enforcement and intelligence agents tasked with keeping us safe. In the wake of 9/11, agencies from the FBI to local police departments have shifted their focus from prosecuting crimes after the fact to interdicting terrorism plots before they are actualized. But if those would-be terrorists share so much in common with “ordinary Americans,” how do you identify those threats?

In practice, this conundrum has led law enforcement to more aggressive surveillance programs and broader use of confidential informants. In the eyes of many civil libertarians, it has also meant illegal monitoring of mosques and law enforcement entrapping terrorism suspects.

Thankfully, Bergen’s book does not fall into tired cliches either defending or excoriating law enforcement. Rather, he humanizes the fear and sense of purpose among counterterrorism professionals in the wake of 9/11, but does not shy from the conclusion that their determination to prevent terrorism has led to unacceptable intrusions on Americans’ civil liberties.

New York City is the biggest target for jihadis hoping to attack the United States and is also central to the debate about appropriate law enforcement techniques. After 9/11, the New York Police Department dramatically enhanced its ability to gather intelligence — both inside the city and beyond — in an effort to interdict any plot that might threaten New Yorkers. The CIA and FBI had failed to “connect the dots” prior to 9/11; New York officials aimed to ensure that no such failure would lead to another major strike in the city they were obligated to defend.

The heart of that effort was the NYPD’s Intelligence Division, which recruited a 30-year veteran of the CIA following 9/11 and employed civilian analysts focused not on gathering evidence of crimes, but on figuring out where the next threat to the city would come from. The Intelligence Division built a broad collection program focused on Muslim communities in New York and surrounding areas. Part of this project was simply an effort to better understand social, economic and demographic conditions, but a Pulitzer Prize-winning Associated Press investigation revealed that it also targeted mosques, bookshops and restaurants where Muslims would gather. The NYPD was effectively monitoring innocent people based on their religion.

These oversteps were real, but they were not the product of jack-booted thugs with discriminatory intent. Rather, Bergen portrays them as the overreaches of everyday Americans living with the day-to-day responsibility of preventing terrorism, and fearful of the consequences if they were to fail. Bergen challenges readers to imagine whether we would do better in their shoes.

The debate over security and liberty in the United States predates our Constitution, but rarely has the balance between those principles been so stark as in the years since 9/11. Bergen’s profile of the (mostly) men accused of supporting jihadi terrorism and those doing everything they can to prevent those attacks reminds that this is a fight of practical choices between otherwise normal people.

Presidential candidates should take notice. The overheated rhetoric of the campaign season, in which Donald Trump irrationally called for banning all Muslims from the United States, is out of whack with the everyday work of keeping the United States safe. Such proposals contradict cherished American principals of religious freedom; just as fundamentally they try to solve the wrong problem. The real danger, Bergen argues, does not primarily come from abroad. The real world challenge is less existential, less dramatic and more insidious, and it starts with “ordinary Americans.”